1. Field of the Invention
The present invention relates to a new and improved construction of an impression cylinder or pressing device of a rotogravure press and which is of the type comprising an elastic tubular-shaped roll jacket or shell. At least one pressure or support element supports this elastic tubular-shaped roll shell upon a central carrier or support, and the support element or support elements exert a pressing force in a pressing plane.
2. Discussion of the Background and Material Information
Such type of impression cylinder or pressing device is known to the art, for instance, from Swiss Patent No. 671,732, granted Sept. 29, 1989 and the U.S. Pat. No. 4,058,877, granted Nov. 22, 1977 and serves, for instance, for the printing of wide material webs in a rotogravure press or printing machine. The material web which is to be printed is pressed by means of the impression cylinder or pressing device against a form cylinder. As a result, the ink or coloring material located in the engraving of the form cylinder can be transferred to the material web, for instance, a paper web which is to be printed, a plastic foil or sheet or some other suitable material which is to be printed. Such impression cylinders possess the advantage that they render possible an essentially uniform pressure impingement throughout the width of the roll defining the impression cylinder and afford a relatively faultless printing of wide material webs having a width up to as much as 10 meters. By using a plurality of pressure or support elements equipped with separate infeed of the pressure medium such impression cylinders can be accommodated to different widths of the material webs to be printed.
It is a well known effect in rotogravure presses that because of the adhesion forces and/or the capillary forces the ink does not always completely depart from the depressions in the engraving, resulting in a poorly printed product In order to eliminate this undesired effect, it is known, for instance, from German Patent No. 2,058,315, published June 24, 1971, to electrostatically charge the surface of the impression cylinder or roll. With the aid of the applied electrostatic charge there can be overcome the capillary forces and all of the ink is completely applied to the material web which is to be printed.
An electrostatic charging technique is known to the art in which the electrostatic charge of the impression cylinder is produced by a corona discharge between an electrode arranged at the rear side of the roll at a small spacing therefrom and the surface of the impression cylinder or roll. This electrode produces at the surface of the cylinder or roll an electrostatic charge. This electrostatic charge is distributed throughout the surface of the cylinder or roll and flows off to the material web to be printed and the form cylinder. Since the electrode is arranged above the impression cylinder, this charging technique is sometimes referred to as "top loading" or "top charging".
There are also known to the art charging systems, for instance, from European Patent No. 115,611, published Aug. 15, 1984, wherein the electrostatic charge is applied from the side through the jacket or shell of the impression cylinder onto the surface thereof. The jacket or shell of the impression cylinder internally comprises, first of all, a layer of a highly insulating material onto which there is applied a layer possessing good electrical conductivity. This layer of good electrical conductivity protrudes, at least at one end of the cylinder or roll, beyond a superimposed electrically poorly conductive soft cover or covering of the cylinder or roll jacket or shell. In this charging system, referred to as "side charging" or "side loading", the protruding part of the highly conductive layer is electrically charged from the side throughout the entire width of the cylinder or roll The electric charge is distributed over the poorer conductive covering of the cylinder or roll jacket or shell and the surface thereof is charged This system possesses the advantage that there are not required any external electrodes, and thus, there is dispensed with the use of a component which requires an appreciable amount of maintenance because of contamination or soiling thereof.
With these methods there is frequently the need to print material webs of different widths with the same printing press or machine and with the same impression cylinder or roll. When attempting to print material webs of reduced width this leads to the problem that the surface of the impression cylinder comes into direct contact with the form cylinder at those locations where there is not present any material web, since the same pressing force prevails over the entire width of the impression cylinder. However, at those locations where there is not present any material web the insulating action of the material web is absent. As a result, the electrostatic charge, upon charging the impression cylinder or roll, flows over the entire cylinder width along the path of least resistance, in other words preferably flows to the uncovered location of the impression cylinder or roll. Thus, at that location where the charge is needed for transfer of the ink to the material web, the electrostatic charge is reduced, and consequently, the printing effect is impaired.
Now in order to avoid this drawback, the impression cylinder known from German Patent No. 2,058,315, published June 24, 1971, possesses conductive layers of different lengths which are alternatingly distributed over the circumference of the impression cylinder. These conductive layers can be separately charged In this way it is possible to selectively connect only wider conductive layers or only narrower conductive layers, and thus, the electric charge can be adjusted to the desired width of the material web. What is disadvantageous with this prior art equipment is that, owing to the intermediate spaces between the conductive layers it is not possible to uniformly charge the circumference of the impression cylinder, rather it only can be charged in discreet zones or strips. Consequently, it is not possible to attain any optimum quality of the printed product. Furthermore, ink particles are increasingly transferred at the uncovered surfaces of the impression cylinder from the form cylinder to the surface of the impression cylinder. Hence, this impression cylinder or roll is more rapidly destroyed at those locations by abrasion and by virtue of the solvent contained in the printing ink.